Surfaces of Revolution with Constant Mean Curvature H = c in Hyerbolic 3-Sace H 3 ( c 2 Kinsey-Ann Zarske Deartment of Mathematics, University of Southern Mississii, Hattiesburg, MS 39406, USA E-mail: kinseyann.zarske@eagles.usm.edu Abstract In this article, we construct surfaces of revolution with constant mean curvature H = c in hyerbolic 3- sace H 3 ( c 2 of constant curvature c 2. It is shown that the limit of the surfaces of revolution with H = c in H 3 ( c 2 is catenoid, the minimal surface of revolution in Euclidean 3-sace as c aroaches 0. Introduction Let R 3 be equied with the metric ds 2 = (dt 2 + e 2ct {(dx 2 + (dy 2 }. (1 The sace (R 3, g c has constant curvature c 2. It is denoted by H 3 ( c 2 and is called the seudosherical model of hyerbolic 3-sace. From the metric (1, one can easily see that H 3 ( c 2 attens out to E 3, Euclidean 3-sace as c 0. In H 3 ( c 2, surfaces of constant mean curvature H = c are articularly interesting, because they exhibit many geometric roerties in common with minimal surfaces in E 3. This is not a coincidence. There is a one-to-one corresondence, so-called Lawson corresondence, between surfaces of constant mean curvature H h in H 3 ( c 2 and surfaces of constant mean curvature H e = Hh 2 c2 [Lawson]. Those corresonding constant mean curvature surfaces satisfy the same Gauss-Codazzi equations, so they share many geometric roerties in common, even though they live in dierent saces. In this article, we are articularly interested in constructing surfaces of revolution with H = c in H 3 ( c 2. Hyerbolic 3-sace does not have rotational symmetry as much as Euclidean 3-sace does. From the metric (1, we see that rotations on the xy-lane i.e. rotations about the t-axis may be considered in H 3 ( c 2. Surfaces of constant mean curvature H = c in H 3 ( c 2 can be in general constructed by Bryant's reresentation formula which is an analogue of Weierstrass reresentation formula for minimal surfaces in E 3 [Bryant]. But it is not suitable to use to construct surfaces of revolution with H = c. We calculate directly the mean curvature H of the surface obtained by rotating an unknown role curve about the t axis. This results a second order non-linear dierential equation of the role curve. We unfortunately cannot solve the dierential equation analytically but are able to solve it numerically with the aid of MAPLE software. Once we obtain the role curve, we then construct surface of revolution with H = c simly by rotating the role curve about the t-axis. From the dierential equation of role curves, it can be seen that the limit of the surfaces of revolution with H = c in H 3 ( c 2 is a catenoid, the minimal surface of revolution in Euclidean 3-sace as c aroaches 0. This limiting behavior of the surfaces of revolution with H = c in H 3 ( c 2 is also illustrated with grahics. The author is a junior mathematics and hysics major of the University of Southern Mississii. This research has been conducted under the direction of Dr. Sungwook Lee in the Deartment of Mathematics at the University of Southern Mississii. 1 Parametric Surfaces in H 3 ( c 2 Let M be a domain and ϕ : M H 3 ( c 2 a arametric surface. The metric (1 induces an inner roduct on each tangent sace T H 3 ( c 2. This inner roduct can be used to dene conformal surfaces in H 3 ( c 2. Denition 1. ϕ : M H 3 ( c 2 is said to be conformal if ϕ u, ϕ v = 0, ϕ u = ϕ v = e ω/2, (2 1
where (u, v is a local coordinate system in M and ω : M R is a real-valued function in M. The induced metric on the conformal arametric surface is given by ds 2 ϕ = e ω {(du 2 + (dv 2 }. (3 In order to calculate the mean curvature of ϕ, we need to nd a unit normal vector eld N of ϕ. For that, we need something like cross roduct. H 3 ( c 2 is not a vector sace but we can dene an analogue 1 of cross roduct locally on each tangent sace ( T H 3 ( c 2. Let v = v 1 t + v ( ( 2 x + v 3 y, ( w = w 1 t + w ( ( 2 x + w 3 y T H 3 ( c 2, { ( where t, ( ( } x, y denote the canonical basis for T H 3 ( c 2. then dened to be v w = (v 2 w 3 v 3 w 2 where = (t, x, y H 3 ( c 2. Let The cross roduct v w is ( t ( + e 2ct (v 3 w 1 v 1 w 3 x ( + e 2ct (v 1 w 2 v 2 w 1 y E := ϕ u, ϕ u, F := ϕ u, ϕ v, G := ϕ v, ϕ v. Then by a direct calculation we obatin (4 Proosition 2. Let ϕ : M H 3 ( c 2 be a arametric surface. Then on each tangent lane T ϕ(m, we have ϕ u ϕ v 2 = e 4ct(u,v (EG F 2 (5 where = (t(u, v, x(u, v, y(u, v H 3 ( c 2. Remark 3. If c 0, (5 becomes the familiar formula from the Euclidean case. ϕ u ϕ v 2 = EG F 2 2 The Mean curvature of a Parametric Surface in H 3 ( c 2 In the Euclidean case, the mean curvature of a arametric surface ϕ(u, v may be calculated by Gauss' formula Gl + En 2F m H = (6 2(EG F 2 1 We will simly call it cross rodcut. where l = ϕ uu, N, m = ϕ uv, N, n = ϕ vv, N and N is a unit normal vector eld of ϕ. It is not certain, but (6 does not aear to be valid for arametric surfaces in H 3 ( c 2 in general. The derivation of (6 requires the use of Lagrange's identity, but it is no longer valid in the tangent saces of H 3 ( c 2. However, (6 is still valid for conformal surfaces in H 3 ( c 2. It is well-known that: Proosition 4. Let ϕ : M H 3 ( c 2 be a conformal surface satisfying (2. The mean curvature H of ϕ is then comuted to be H = 1 2 e ω ϕ, N. (7 One can then easily see that the the formulas (6 and (7 coincide for conformal surfaces. 3 Surfaces of Revolution with Constant Mean Curvature H = c in H 3 ( c 2 In this section, we construct a surface of revolution with constant mean curvature H = c in H 3 ( c 2. As mentioned in Introduction, rotations about the t-axis are the only tye of Euclidean rotations that can be considered in H 3 ( c 2. Consider a role curve α(u = (u, h(u, 0 in the tx-lane. Denote ϕ(u, v as the rotation of α(u about the t-axis through an angle v. Then, ϕ(u, v = (u, h(u cos v, h(u sin v. (8 The quantities E, F, and G are calculated to be E = e 2cu {e 2cu + (h (u 2 }, F = 0, G = e 2cu h(u. If we require ϕ(u, v to be conformal, then e 2cu + (h (u 2 = (h(u 2. (9 The quantities l, m, n are calculated to be h (uh(u l = (h(u2 (e 2cu + (h (u 2, m = 0, n = (h(u 2 (h(u2 (e 2cu + (h (u 2. 2
The mean curvature H is comuted to be 2. H = Gl + En 2F m 2(EG F 2 = 1 h(uh (u + e 2cu + (h (u 2 2 e 2cu (e 2cu + (h (u 2 (h(u 2 (e 2cu + (h (u 2. If we aly the conformality condition (9, H becomes H = h (u + h(u 2e 2cu (h(u 3. (10 Let H = c. Then (10 can be written as h (u h(u + 2ce 2cu (h(u 3 = 0. (11 Hence, constructing a surface of revolution with H = c comes down to solving the second order nonlinear dierential equation (11. If c 0, then (11 becomes h (u h(u = 0 (12 which is a harmonic oscillator. This is the role curve for a surface of revolution in E 3. (12 has the general solution h(u = c 1 cosh u + c 2 sinh u. For c 1 = 1, c 2 = 0, ϕ(u, v is given by ϕ(u, v = (u, cosh u cos v, cosh u sin v (13 This is a minimal surface of revolution in E 3, which is called a catenoid since it is obtained by rotating a catenary h(u = cosh u. See Figure 1. Unfortunately, the author cannot solve (11 analytically, so we solve it numerically with the aid of MAPLE. References Figure 1: Catenoid in E 3 [Lawson] H. Blaine Lawson, Jr., Comlete minimal surfaces in S 3, Ann. of Math. 92, 335-374 (1970 [Bryant] Robert L. Bryant, Surfaces of mean curvature one in hyerbolic 3-sace, Astérisque 12, No. 154-155, 321-347 (1988 4 The Illustration of the Limit of Surfaces of Revolution with H = c in H 3 ( c 2 as c 0 In section 3, it is shown that the limit of surfaces of revolution with constant mean curvature H = c in H 3 ( c 2 is a catenoid, a minimal surface of revolution in E 3. Such limiting behavior of surfaces of revolution with H = c in H 3 ( c 2 is illustrated with grahics in Figure 2 (H = 1, Figure 3 (H = 1 2, Figure 4 (H = 1 4, Figure 5 (H = 1 8, Figure 6 (H = 1 64, Figure 7 (H = 1 256. Figure 7 (b already looks retty close to the catenoid in Figure 1. 2 The validity of this formula should not be a concern since we assume that the surface is conformal. 3
(a Prole curve h(u (a Prole Curve h(u (b Surface of Revolution in H 3 ( 1 Figure 2: Constant Mean Curvature H = 1 (b Surface of Revolution in H 3 ( 1 4 Figure 3: Constant Mean Curvature H = 1 2 4
(a Prole Curve h(u (a Prole Curve h(u (b Surface of Revolution in H 3 ( 1 16 Figure 4: Constant Mean Curvature H = 1 4 (b Surface of Revolution in H 3 ( 1 64 Figure 5: Constant Mean Curvature H = 1 8 5
(a Prole Curve h(u (a Prole Curve h(u (b Surface of Revolution in H 3 ( 1 4096 Figure 6: ConstSurface of Revolution in H 3 ( c 2 ant Mean Curvature H = 1 64 (b Surface of Revolution in H 3 ( 1 65535 Figure 7: Constant Mean Curvature H = 1 256 6